It is known to provide optically variable devices in which arrays of lenticular (part-cylindrical lenses) focus on an object plane containing multiple sets of interleaved image elements. Each set of image elements (strips) belongs to a distinct image, so that as the person viewing the device changes the angle of view, a different image becomes visible.
In security applications, and in particular when dealing with flexible security documents such as banknotes, it is desirable to minimise the thickness of a lens array applied to the security document. For example, a thickness for polymer banknote substrates currently in use is approximately 90 microns, including the thickness of the lens array. In order to meet this design constraint, lenses of diameter approximately 50 microns or less are used.
The effect produced by optically variable devices containing multiple sets of interleaved image elements as described above is sometimes known as a “flipping image” effect. The number of distinct images in the flipping image effect is limited by the number of sets of image elements that can be placed in the field of view of a lens in the lens array. For example, if a two-channel flipping image is to be produced, then two sets of image elements are required. This means that each image element can have a width no greater than half the width of a lens.
If lenses of width 50 microns are used, the image elements should be no greater than 25 microns wide in order to ensure that there is minimal cross-talk between the channels of the flipping image. An image element width of 25 microns or less is achievable with some techniques used in security printing. However, other commonly used techniques, such as gravure (sometimes known as rotogravure) printing cannot consistently apply image elements having this width. A minimum practical width of line elements currently achievable with gravure printing is approximately 35-45 microns. Image elements of this width produce unacceptably large amounts of cross-talk when used with 50 micron diameter lenses.
It has previously been found that a given substrate thickness can be maintained, while increasing the diameter of the lenses in the lenticular array, by adjusting the lens parameters such that the focal point width of the lenses in the object plane is approximately the same as the width of the image elements, as described in our PCT application PCT/AU2010/000243, incorporated herein by reference in its entirety. For example, for a substrate thickness of 90 microns, lenses of diameter 63.5 microns can be used. However, even with lenses of this increased diameter, a flipping image device with two channels still produces unacceptable cross-talk because the minimum practically achievable width of gravure line elements is 35-45 microns, which is still more than half the lens diameter.
There is, therefore, a need for an optically variable device which can produce flipping image effects, and which can be produced using a wider variety of security printing techniques, while being less susceptible to cross-talk.